The present invention relates to a method of aircraft traffic indication, and more particularly, to a method of aircraft traffic surveillance for analysis to avoid potential collision with other aircraft.
The increased demands placed on the aircraft flight deck as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring of aircraft traffic in a vicinity of an aircraft that includes automatic identification of potential threats to the monitoring aircraft As a result, aircraft can have transponders associated therewith that, in response to appropriate electromagnetic interrogation signals provide responding electromagnetic signals that include information with respect to the range, altitude, and bearing of the interrogated aircraft. Certain traffic control system transponders, e.g., the mode-S system, include target identification as part of the information imposed on the responding electromagnetic signals. For these mode-S systems, the identification of the path or track of the responding aircraft is relatively simple, involving time dependent positions and altitudes of an identified aircraft. Similarly, extrapolations or extensions of aircraft tracks can be relatively simple.
In systems (such as the Air Traffic Control Radar Beacon System, ATCRBS) which do not include unique aircraft identification information, the determination of the aircraft tracks is more complicated. The information obtained by periodic interrogation of a multiplicity of unidentified targets, with associated range, altitude and bearing information being provided or determined as a result of the interrogation, can be subjected to well known algorithms to provide a target aircraft track. Once the track is identified, then the extension thereof can be computed to determine if the target aircraft is a threat to the monitoring aircraft.
The track determination is complicated for several reasons generally involving spurious target images. For example, a monitoring aircraft can transmit an interrogation signal to a target aircraft, whereupon a transponder in the target aircraft provides a first response signal (direct reply), the delay between the transmission of the interrogation signal and the reception of the first response signal providing the range information. However, the interrogation signal can result in a second response signal that is reflected from the earth's surface. The second response signal, reflected once from the earth, is generally referred to as a single reflection multipath (or type I) reply. Because the length of time for the travel of the second response signal is longer than for the first response signal, the second response signal can be interpreted as a separate target aircraft at a greater range from the monitoring aircraft. Type I or single reflection multipath also can be generated by an interrogation which reflects off the ground combined with a direct reply. Since the path length is the same as the previous case, the range is the same. Similarly, an interrogation signal can reflect off the surface of the earth, activate the transponder of the target aircraft which provides a response signal that also reflects off the earth's surface. In this instance, since both the interrogation signal and the second response signal are each reflected once from the earth's surface, this reply is referred to as a double reflection multipath (or type II) reply. This response signal will be interpreted by the monitoring aircraft as a target aircraft at an even greater range than indicated by the direct or type I reply. In this situation, a single target aircraft is providing the monitoring aircraft with a plurality of target responses during each interrogation period. Thus, from a single interrogation cycle (consisting of more than one interrogation), up to three responses can be received from a single target aircraft; namely, a direct reply, a type I reply, and a type II reply. These three tracks are referred to as the normal (direct reply) track, and the image tracks (from type I and II multipath replies).
Tracks also can be formed on mixtures of reply types. In particular, tracks can be formed on combinations of single and double reflection replies. Since the normal track is generally very steady with normal replies available in nearly every scan, tracks formed on a combination of normal replies and reflected replies typically are not formed. However, if a track is lost and reformed under multipath conditions, tracks based on combinations of normal and multipath replies could be formed. In addition, established multipath tracks can be updated by replies generated by the other class of multipath, i.e. a type I multipath track could be updated with a type II multipath reply or vice versa. These updated tracks then in many cases are reclassified as non-multipath tracks. It is also possible to update a direct track with a multipath reply if the direct reply is missing. These effects can occur any time multipath exists. However, the effect is most noticeable at ranges, altitudes, and altitude differences where the elevation angle between the monitoring aircraft and the target aircraft are such that all types of replies can be missing. For systems (using r-trackers) which use two replies to initiate the tracking filters, additional tracks can be formed on combinations of direct, type I, and type II replies, or 6 additional tracks, and for systems (using r.sup.2 -trackers) which require three successive replies from three successive interrogations to initiate a track, 24 additional tracks can be formed on the combination of replies (i.e., in addition to the direct, type I, and type II tracks). These 6 (or 24) additional tracks are referred to as mixed multipath tracks.
False tracks are a distraction to the pilot when displayed Thus, there is a need to reduce the number of false multipath (and mixed multipath) tracks which are formed and displayed by the TCAS systems. The method of the present invention includes novel techniques which prevent false tracks from being established, and multipath and mixed multipath tracks that are initiated are tagged as such until they coast out or are dropped. The method of the present invention for forming and updating tracks including the reduction of multipath tracks is accomplished by identification of image tracks, classification of tracks and transitions between classes of tracks, special handling of various classifications of tracks, and utilizing a novel predetermined track update order. Also, tracking algorithm selection (r.sup.2 tracker) helps in reducing false tracks for all intruders.
In existing systems, tracks are updated in increasing range order without any consideration to track classification in determining the update order. Test results indicated that newly formed image tracks could steal replies from long existing normal tracks. This led to two problems--the normal track would either coast or be updated with some other reply, leading to split tracks, and the image track would be updated with a non-image reply, leading to failure of the image criteria and establishment of the track as a normal (non-image) track. The method of the present invention reduces these negative effects by giving the established, normal tracks first review at the data. The method of the present invention updates tracks in range order from shortest range to longest range within a classification, the order of the classification of the method of the present invention being established normal tracks, non-established image tracks, and potential image tracks.